Shulou(Shulou.com)06/01 Report--

OSPF: link-state routing protocol, metric: cost

Features: adapting to large-scale networks, fast convergence of routing changes, no routing loops, support for variable length subnet mask (VLSM), support for area division, support for sending protocol messages at multicast addresses

How it works: first, "establish a connection relationship"-- list other routing information in the network by learning link-state information, generate a "link-state database"-- then calculate the best path (shortest path tree) to each router according to the Dijkstra algorithm-- and finally, each router constructs its own "routing table" from the shortest path tree.

Identification of routers in Router ID:OSPF network

Selection method: first, the router selects the IP address with the highest value on all its Loopback interfaces as the Router ID. If the IP address of the Loopback interface is not configured, the router selects an IP address with the highest value as its Router ID among all the active physical interfaces. It is recommended to use the Loopback interface as the Router ID because the Loopback interface is more stable than the physical interface.

DR and BDR: because routers running OSPF pass link-state information to each other by establishing adjacencies with neighboring routers As a result, each router has to transmit link-state information with all other neighboring routers, which will lead to confusion. In order to solve this unnecessary waste of network resources, you can elect a designated router to establish an adjacency with other routers in the network and be responsible for telling them about the changes in the network segment. The selected router is called DR, and BDR is the backup of this DR. If the DR fails in the network, this BDR will become the new DR.

The selection of DR and BDR is divided into two types: automatic and manual. Automatic election will have the largest Router ID on the network segment, DR, and the second largest manual election for BDR; requires setting the priority of the router. The interface mode command is ip ospf priority 0-255. the higher the value, the higher the priority, the Cisco router defaults to 1. If set to 0, the router does not participate in the election.

The steps of establishing OSPF adjacency are divided into seven steps.

1. Down state: initialization state in which the router does not exchange information with any neighbors

two。 Initial (init) status: each interface of the router sends Hello messages through 224.0.0.5 multicast

3. Bidirectional (2-way) status: the router sends an Hello echo message containing its own ID, election DR, and BDR

4. Quasi-start (exstart) status: establishing adjacency

5. Exchange status: a live multiple database description package (DBD) is exchanged between master / slave routers, containing summary information of LSA entries

6. Load (loading) status: synchronous updates, including link-state request packets (LSR). Link-state update packets (LSU) respond to LSR after the request, and link-state update packets (LSU) need confirmation from the other party.

7. Perfect adjacency (full) status: complete database synchronization between routers, which will be achieved when they are completely consistent.

Lab: Cisco Router configuring OSPF

The steps of the experiment:

1. Configure IP addresses for PCs and routers

two。 Configure 3 router OSPF. Take R1 as an example.

3. View the routing table and verify the lab

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